Persistently cache InputAccessor::fetchToStore()

This avoids repeated copying of the same source tree between Nix
invocations. It requires the accessor to have a "fingerprint" (e.g. a
Git revision) that uniquely determines its contents.

src/libfetchers/mercurial.cc

@@ -339,6 +339,14 @@ struct MercurialInputScheme : InputScheme
 
         return makeResult(infoAttrs, std::move(storePath));
     }
+
+    std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override
+    {
+        if (auto rev = input.getRev())
+            return rev->gitRev();
+        else
+            return std::nullopt;
+    }
 };
 
 static auto rMercurialInputScheme = OnStartup([] { registerInputScheme(std::make_unique<MercurialInputScheme>()); });